This disclosure relates to a method for heating thermoplastic pre-forms, such as parisons, by a radiant energy source prior to a blow molding operation.
Plastic pre-forms and parisons have been heated prior to blow molding operations in the prior art typically by placing the parisons adjacent electrical heating bands or coils. This arrangement produced at least four undesireable results. First, a relatively high heat differential or gradiant was established across the thickness of the parison wall, which is undesireable during the blow molding operation. Second, in order to bring the inner portion of the parison to a blowable temperature, the outer parison surface was often overheated. This was completely unacceptable for certain types of materials, such as polyethylene terephthalate which crystallizes at elevated temperatures. Third, the heat distribution along the axis of the parison was erratic because of localized hot segments nearest each electrical band, resulting in undesireable thinned or thickened sections in the final blown article. Fourth, the electrical heating bands did not satisfactorily vary the parison temperature along the axis of the parison to control the thickness in the final article, because the bands produced sharp transition temperatures along the axis rather than smooth temperature gradiants.
Several attempts have been made to overcome these prior art problems, including the use of various radiant heating means.
One such radiant heating includes a nicrome heating element within a quartz envelope. This particular heating element did not sufficiently heat the inside surface of the parison, apparently because the infra-red energy waves did not sufficiently penetrate the parison wall. In order to adequately heat the interior of the parison, it became necessary to extend the time period that the parison was exposed to the heating element. This, however, resulted in a loss of cycle time in the overall operation and overheated the parison at its outer surface. Additionally, this arrangement did not provide a smooth temperature gradiant between differentially heated segments of plastic parisons.